Value of Output Cathode resistor

[rusty-jade]

I built the Stereomour as a 2A3 amp but modified it to a 45 power amp with the Triad Cx-7 chokes. I am thinking of replacing the two 1.6K output cathode resistors with Mills wirewound non-inductive resistors, because I think they are better but more important, because I am getting a reading of 2.62 volts between pins 1 and 4 of the two 45 tubes.  ( Part of the problem could be my ac outlet voltage which is  close to 124 volts) I cannot find any 1.6K mills 12W resistor, but there are values of 1.5K and 1.8K ( 12W).  I am thinking of using 1.8K resistor. Has anyone upgraded the cathode output resistors and what values did you use?   

 

[vetmed]

The value of the resistors has nothing to do with the voltage you are getting. What you are measuring is the heater voltage, likely slightly high because your wall voltage is on the high side, and because your 45s draw less heater current than the 2A3s. Do NOT change the value of the cathode resistor, it will not change the filament voltage, and it WILL alter your operating point, not necessarily for the better

Regards
      Robert Lees

 

[Paul Joppa]

Vetmed is correct.

If you chance the resistor anyhow  :^)  I'd use the higher resistance 1.8K which will slightly reduce the plate current. Because your plate voltage is probably also high, reducing the current keeps the dissipation closer to nominal. The upshot is slightly more power and slightly higher distortion at that higher power. I doubt anyone can hear the difference, it's small.

The cathode resistor is bypassed with a large cap on the PSU board. I also doubt that changing the resistor makes much sonic difference; I'd do the cap before bothering with the resistor. Use 47uF or more, 100 volts or more.

 

[rusty-jade]

Thanks to both.  I think I will leave the stock resistor alone.  I omitted to mention that I had already bypassed the said electrolytic cap way back with a 47uf film cap when I installed the CX-7 chokes.

 

[rusty-jade]

Another thing which may be of interest to some Stereomour owners.  I have tried both the Triad CX-7 and the Hammond 154G chokes.  Because the 154G is much smaller, they can be located near the corner ends while that is not possible with the CX-7 ( I have substituted very large parafeed and coupling caps). There is a slight white noise ( from RFI I guess), independent of volume, which is far less with the 154G - you have to put your ears next to the speakers to hear it ( Tekton Lore)    This noise is  louder with the CX-7, but not audible once music starts. However, sonically, the CX-7 is much better with a larger sound staging and one can hear musical instruments much clearer and louder.  After a week, I switched back to the CX-7s.  With the noise though, if one were to use a powered sub, it may require connecting the sub separately with a splitter from the pre-amp rather than from the Stereomour to the powered amp sub and then the speakers.

 

[Grainger49]

Rusty,

Noise that can only be heard with your ear to the speaker is not significant.  You don't listen in that position. 

I recently heard such a noise and have not even bothered to find its source.

 

[rusty-jade]

Yes I agree.  I was trying to point out that for those who have not installed the choke, the CX-7 is better sonically.  And also if someone wishes to complement his/her system with a powered sub, ( I have toyed with that idea, from time to time - for jazz and vocals the 45 is superb, better than the 2A3, in my view and far more than some push-pull tube amps I have owned but not always so for classical music) that noise could be amplified if the connection is from the Stereomour to the powered sub, then the speakers, rather than connecting from the preamp to the sub and Stereomour separately with Y splittered cables.  I have no idea which connection method is better; only that synchronization of a powered sub to the system is not easy as I returned one sub some time ago to a distributor.  Thanks for your comment.
Cheers!